The origin of adhesion at nanoscale contacts in humid air is investigated by pull-off force measurements using atomic force microscopes in controlled environments from ultra-high vacuum through various humidity conditions to water. An equivalent work of adhesion (WOA) model with a simplified interface stress distribution is developed, combining the effects of screened van der Waals and meniscus forces, that describes adhesion in humid air and which self-consistently treats the contact stress and deformation. Although the pull-off force is found to vary significantly with humidity, the equivalent WOA is found to be invariant. Increasing humidity alters the nature of the surface adhesion from a compliant contact with a localized, intense meniscus force to a stiff contact with an extended, weak meniscus force.
Pub Type: Journals